ADR-0019 §9 sweep (2/2): help blocks + modals + system notes

Final pass of the i18n migration sweep. Every user-visible
string in `src/` now flows through the catalog via `t!()`.

## Categories migrated in this commit

- **help.cli_banner** — the entire `cli::HELP_TEXT` const,
  formerly a 40-line `&'static str`, is now a YAML block in
  the catalog. The const is replaced by a thin
  `cli::help_text() -> String` wrapper that performs the
  catalog lookup. `main.rs` calls `help_text()` for both
  `--help` output and the args-parse error path. The two
  integration tests that referenced `HELP_TEXT` directly are
  updated.
- **help.in_app_body** — the in-app `help` command's body is
  one YAML block; `note_help` becomes 5 lines that iterate
  the lines and emit each as its own output row (preserving
  the renderer's "one logical line = one display row"
  invariant for accurate scroll math).
- **modal.*** — load picker, rebuild confirm, and save-as
  path-entry strings: rebuild_cancelled, load_cancelled,
  generic_cancelled, load_picker_nothing,
  path_entry_empty_name, path_entry_empty_path.
- **dsl.failed** — the `"<verb> <subject>" failed: <rendered>`
  wrapper around the friendly-error layer's translated
  message.
- **dsl.running** — the `running: <input>` echo line shown
  above each command's response. (Note: the en-US prefix
  "running: " is hardcoded in the parse-error caret-padding
  calculation. Translators changing the prefix must keep the
  width consistent — documented inline.)
- **advanced_mode.not_implemented** — the placeholder echo
  shown when SQL hits the unimplemented advanced-mode path
  (Q1 territory).
- **fatal.persistence** — the FATAL banner for
  PersistenceFatal events (ADR-0015 §8).
- **project.{load_path_missing,saveas_target_exists,**
  **import_zip_missing}** — runtime-side project-switch
  validation errors that surface via ProjectSwitchFailed.

## Catalog start-up ordering

`main.rs` now calls `friendly::catalog()` at the very top
(before args parsing) so `help_text()` works in both the
success path and the args-error path. A corrupted build
artefact still fails loudly with a useful panic; the
practical risk is essentially zero since the catalog is
`include_str!`'d at compile time and validated by the unit
test before shipping.

## Remaining literals

The only `note_*` calls in `src/` that still pass plain
strings are inside `#[cfg(test)]` modules — synthetic test
fixtures, not user-visible. The codebase passes the "every
user-visible string flows through the catalog" bar.

## Tally

610 tests passing (no change in count — pure refactor).
Clippy clean with nursery lints.

## What this closes

ADR-0019 §9 (migration sweep) — done.

ADR-0019 itself is now fully implemented:
- §1-§5: catalog + translator + voice + verbosity ✓ (`eac7e5b`)
- §6: row pinpointing + schema enrichment ✓ (`431645a`)
- §9: migration sweep ✓ (this + `aff528a`)
- §10: anchor phrases preserved throughout ✓
- The five "Out of scope" items remain explicitly bounded
  to future ADRs (advanced-mode SQL, settings persistence,
  pluralisation, runtime locale, value formatting,
  constraint management).

src/cli.rs

@@ -31,51 +31,16 @@ pub struct Args {
     pub help: bool,
 }
 
-/// Usage banner printed by `--help`. Kept as one block so the
-/// formatting is reviewable on its own.
-pub const HELP_TEXT: &str = "\
-rdbms-playground — a TUI playground for relational database concepts
-
-Usage:
-    rdbms-playground [options] [<project-path>]
-
-Arguments:
-    <project-path>           Path to an existing project directory.
-                             Without this, a fresh auto-named temp
-                             project is created in the data dir.
-
-Options:
-    -h, --help               Print this help and exit.
-        --theme <light|dark> Override theme (default: auto-detect).
-        --data-dir <PATH>    Use PATH as the data root instead of
-                             the OS-standard location for this run.
-        --log-file <PATH>    Write tracing output to PATH.
-        --resume             Open the most-recently-used project
-                             (path tracked under <data-root>/last_project).
-                             Errors out if no previous project is
-                             recorded. Mutually exclusive with
-                             <project-path>.
-
-App-level commands (typed inside the app, available in both modes):
-    quit / q                 Exit cleanly.
-    mode simple|advanced     Switch input mode.
-    help                     Show this list of commands in-app.
-    save                     Save the current temp project under a
-                             chosen name (or `save as` to copy a
-                             named project to a new location).
-    save as                  Always prompt for a target name/path.
-    new                      Close current, create a fresh temp.
-    load                     Open the project picker.
-    rebuild                  Rebuild playground.db from project.yaml
-                             + data/, with confirmation.
-    export [<path>]          Write a zip of project.yaml + data/ to
-                             <path> (relative paths under the data
-                             root). Excludes playground.db and
-                             history.log.
-    import <zip> [as <t>]    Unpack <zip> into a new project and
-                             switch to it. <t> overrides the target
-                             name (else taken from the zip).
-";
+/// Usage banner printed by `--help`.
+///
+/// Wraps the catalog lookup (`help.cli_banner`) so callers
+/// don't have to spell out the key. The catalog body is the
+/// single source of truth — see
+/// `src/friendly/strings/en-US.yaml`.
+#[must_use]
+pub fn help_text() -> String {
+    crate::t!("help.cli_banner")
+}
 
 #[derive(Debug, thiserror::Error)]
 pub enum ArgsError {