chore(i18n): drop tabs.home, the home tab's own title key

[PathGao]

Prerequisite

Third in a chain — merge #480, then #482, then this:

master ← #480 (Ctrl+T means new file)
           ← #482 (remove addHomeTab, keep the sentinel)
                ← this PR (remove tabs.home, the title that tab carried)

Branched on and targeting chore/drop-dead-home-tab-constructor, so the diff
above is this change alone. GitHub retargets each base automatically as the one
below it lands.

This is the follow-up #482's Scope section offered.

What this is

tabs.home was the home tab's own title key: addHomeTab set
title: t('tabs.home', settings.language), and that was its only consumer in
src/. #482 removed the method, so the key now titles nothing.

26 locales, one leaf each, always the second entry of a tabs block that keeps
untitled:

         tabs: {
-            untitled: 'Untitled',
-            home: 'Home'
+            untitled: 'Untitled'
         },

Scope

The home screen is not affected. There are three home keys per locale and
this touches one:

key	what it is	here
menu.home	the Home menu item / titlebar button label	kept
home.*	the HomePage section (welcome, recent files, …)	kept
tabs.home	the title of the tab that held the home screen	removed

The home screen keeps both of its live routes — the titlebar button and the
showHome toggle. What went is the title of a tab that can no longer exist.

78 → 52 occurrences of home: in the dictionary, which is 26 × 3 → 26 × 2.

Tests

homeTabRender.test.ts read the key to prove the render gate ignores the tab
title in every language — the dead half of the #392 gate compared against the
English literal 'Recents'. It now reads menu.home, which every locale also
defines. The claim under test is that the title cannot affect which branch runs,
not which key the title came from; the following 'Recents' assertion is
unchanged.

Verification

Nothing type-checks a t() key — Translation is an index signature
([key: string]: string | Translation), so svelte-check has no opinion here
and a half-finished sweep across 26 locales would compile fine. The safety net
is i18nCoverage.test.ts, and rather than assume it covers this I broke it both
ways and watched it fail:

A locale left behind. Restored home: 'ホーム' to ja only:

✖ no language defines a key English does not have
    tabs.home  [1] ja

A call site missed. Put t('tabs.home', settings.language) back into
tabs.svelte.ts:

✖ every key the source asks for exists in English
✖ t() never echoes a key back for a reachable string

Both directions of that file's interlock hold, so neither failure mode can reach
master. Probes reverted; the diff is the 26 deletions and the one test line.

Full suite, macOS (darwin 25.5.0), on top of #482:

npm audit        found 0 vulnerabilities
npm run check    649 FILES 0 ERRORS 0 WARNINGS
npm test         737 tests, 737 pass, 0 fail
cargo test       157 passed; 0 failed   (unchanged from #482 — no Rust here)

Not verified: no running app. This is a dictionary deletion whose safety
rests on the two probes above plus the caller census (grep -rn 'tabs\.home' src scripts returns only the comment explaining the substitution), not on switching
the UI through 26 languages by hand.