round-5 follow-up: completion + i18n sweep

Four user-reported gaps from the round-4 testing pass:

1. Empty-prompt hint reworded from "(no active hint)" to
   "Type a command — press Tab for options, `help` for a
   list" (6 snapshots updated to reflect 80-col truncation).

2. App-lifecycle commands (quit/q, help, rebuild, save/save as,
   new, load, export, import, mode, messages) now flow through
   the DSL parser:
   - 15 new keywords + catalog token entries
   - new Command::App(AppCommand) AST with 11 variants
   - parse-first dispatch in submit() (app commands work in
     both simple and advanced modes)
   - pre-chumsky source-slice for `export <path>` /
     `import <zip> [as <target>]` mirrors the replay precedent
   - UsageEntry registry entries so parse errors surface
     relevant usage templates
   - `mode <bad>` / `messages <bad>` use try_map for the
     friendly "unknown mode/messages" wording

3. DSL completion gaps:
   - `1:n` surfaces as a composite candidate at `add `
   - --all-rows / --create-fk / --force-conversion /
     --dont-convert surface as new CandidateKind::Flag
     candidates (coloured with tok_flag in hint panel)
   - filter_clause .labelled() wrap removed so chumsky's
     expected-set surfaces the constituent options

4. Hardcoded user-facing strings migrated to catalog:
   - 4 parser custom errors (incl. the known "tables need at
     least one column" wart)
   - UnknownType Display now via parse.custom.unknown_type
   - UI panel titles + mode labels (Output / Hint / SIMPLE /
     ADVANCED / Advanced:)
   - app.rs cascade rendering (action labels + summary)
   - runtime --resume CLI stderr
   - db.rs change-column diagnostic tables (7 headers + 3
     wrapper summaries + force-conversion hint)

Tests: 765 → 769 passing, 0 failed, 1 ignored (same doctest
as before). Clippy clean with -D warnings.

Deferred:
- ~25 thiserror #[error] attributes still hand-rolled
  (DbError, ArgsError, ArchiveError, PersistenceError,
  LockError). Tracked separately.
- DSL/SQL relationship in advanced mode — clarified
  implicitly via parse-first dispatch; broader ADR
  amendment to follow.
- Post-complete-parse completion gap (e.g. `save ` Tab
  can't offer `as` because `save` parses bare; same shape
  as `--create-fk` after a complete `add relationship`).

src/snapshots/rdbms_playground__ui__tests__default_simple_dark.snap

@@ -1,6 +1,5 @@
 ---
 source: src/ui.rs
-assertion_line: 404
 expression: snapshot
 ---
 ╭ Tables ──────────────────╮╭ Output ──────────────────────────────────────────╮
@@ -23,7 +22,7 @@ expression: snapshot
 │                          ││                                                  │
 │                          │╰──────────────────────────────────────────────────╯
 │                          │╭ Hint ────────────────────────────────────────────╮
-│                          ││(no active hint)                                  │
+│                          ││Type a command — press Tab for options, `help` for│
 ╰──────────────────────────╯╰──────────────────────────────────────────────────╯
 Project: Term Planner                                                           
 Enter submit  ·  : advanced once  ·  mode advanced switch  ·  Ctrl-C quit