feat(dictionary): faster search, browse-by-letter, a11y and share fixes

[MadhuS-1605]

Closes #76

What this changes

Search performance (search.ts, useSearch.ts)

• Phonetic-sibling shards are now lazy. searchKannada loads only the query's own shard(s) up front. It widens to phonetic-neighbour letters (ಸ → ಶ, ಷ) only when the direct exact/prefix lookup finds nothing and the query is 2+ aksharas. An exact hit no longer triggers a multi-shard download.
• Debounce 120 ms → 220 ms, so a fast typist doesn't fire a shard fetch on every keystroke.

Browse by letter (new DictionaryLetterIndex.tsx)

• A varnamale strip (13 vowels + 34 consonants + 2 archaic) in the empty state.
• Per-letter counts summed from manifest.shards; letters with no entries are disabled.
• Tapping a letter seeds a prefix search.

Results UX (SearchResults.tsx, DictionarySearch.tsx)

• SearchResults no longer remounts on every keystroke (dropped key={normalise(q)}); the "show more related" expansion is reset with an effect instead, so it stops collapsing mid-scroll.
• New live role="status" / aria-live="polite" line: {n} results or "no words found" — the no-results state was previously never announced to screen readers.

Share / URL correctness (EntryCard.tsx)

• Copied links and citations now use CANONICAL_ORIGIN instead of window.location.origin, so links copied from a preview deploy no longer carry the preview host.

Page layout (dictionary/page.tsx)

• The offline "download dictionary" button moves below the search UI, behind a divider — it no longer sits above the search box before the user has done anything.

Carried on this branch (adjacent, prior session work)

• Share card: word cards render up to ~5 sense lines instead of just defs[0] (shareCard.ts gains supportMaxLines; the headword + senses block is centred together).
• Proverbs: browse gaade by first letter — a full varnamale index with per-letter counts (features/proverbs/lib/letters.ts, ProverbsBrowse.tsx).

Testing

• tsc --noEmit clean.
• vitest run — 528 tests pass, including a new case in search.test.ts locking in "sibling shards fetched only on a miss", and new tests for the proverb letter index and the multi-line share card.

[MadhuS-1605]

Rebased/merged main into this branch and resolved the conflicts (merge commit a695cd3, DCO signed).

Conflicting files: src/app/dictionary/page.tsx, src/features/dictionary/components/DictionarySearch.tsx, src/features/dictionary/components/EntryCard.tsx, src/features/proverbs/components/ProverbsBrowse.tsx.

Resolution kept main's coral & sky redesign structure and layered this branch's changes on top:

• DictionarySearch — a11y result-count live region + browse-by-letter index in the empty state, on top of the redesigned layout; DownloadDictionaryButton now lives inside the component (from main).
• EntryCard — adopted main's useEntryCopy refactor; re-applied the multi-definition share card (shareSupport, supportMaxLines: 5).
• ProverbsBrowse — took main's grouped letter rail; this branch's flat letter-filter (lib/letters) was superseded and removed.
• dictionary/page.tsx — took main's layout.

npm run typecheck clean, npm test 569 passing. PR now shows mergeable.