fix(i18n): resolve 48 t() calls referencing missing EN locale keys

[krisarmstrong]

Summary

Stem had 48 t() call sites referencing keys that didn't exist in
the EN locale JSON. i18next's fallback semantics (second arg to
`t()`) showed the key as English text so users didn't see broken
UI — but ES translations never resolved, leaving the Spanish
version permanently showing English for these labels.

Surfaced by `check-keys.py` landing via PR #327; cleanup of the
backlog so the `--ratchet` flag can eventually be dropped from CI.

Two-pronged fix

(1) Namespace prefix corrections (24 call sites)

Keys actually exist in `settings.json` but t() calls referenced them
via the default `common` namespace. Fixed by changing literal call
sites to use the `settings:` cross-namespace prefix:

```diff
-t('settings.interface.title', 'Interface')
+t('settings:interface.title', 'Interface')
```

Affected: SettingsDrawer, HeaderInterfaceSelector, InterfaceSection,
ModeSection, ReflectorSection, RFC2544Section, RFC2889Section,
RFC6349Section, Y1564Section, Y1731Section, MEFSection,
TrafficGenSection, TSNSection.

(2) Missing keys added to locale JSON (24 new keys + ES)

common.json:

• `accessibility.{closeSettings, openHistory, refreshInterfaces, selectProfile}`
• `interface.{networkInterfaces, noInterfaces, select}`
• `profile.{current, manage, noProfiles, select}`
• `tooltips.header.{refresh, history, help, settings, logout}`
• `status.{clickToReconnect, tapToReconnect, tapToReconnectHint}`

settings.json:

• `viewModuleHint`, `viewStandardHint`
• `tests.{rfc2544, rfc2889, rfc6349, y1564, y1731, mef, trafficgen, tsn}.title`
  (8 test category headings)

setup.json:

• `buttons.copyTooltip`

ES translations follow `I18N_TRANSLATION_MEMORY.md` (formal `usted`,
sentence case, glossary terms preserved verbatim — Reflector, MEF,
RFC numbers, Y.1564 etc. stay English).

Test plan

• `tsc --noEmit` — clean
• `biome check` — clean
• `vitest run` — 136 passed (no regressions)
• `check-keys.py` strict — was 48 errors, now 0 ✅
• `./scripts/i18n/validate.sh --ratchet` — OK with 4 warnings
  (pre-existing fallback patterns + heuristic JSX flags + version
  pin)

Follow-up

• The 355 `t('key', 'fallback')` patterns the validator still warns
  about are a separate concern (task Docs mismatch: Implementation spec module paths differ from code layout #45 sibling).
• Drop the `--ratchet` flag from
  `.github/workflows/ci.yml`'s `check_key_usage` invocation in a
  separate PR once the fallback-pattern cleanup also lands.
• The 329 same-pattern bugs in seed (PR #1203 surfaced them) will
  get the same fix in a sibling PR.

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License Compliance Report

All dependencies pass license compliance checks

Go Dependencies

• Unknown: 31 package(s)
• MIT: 26 package(s)
• BSD-3-Clause: 16 package(s)
• Apache-2.0: 11 package(s)
• BSD-2-Clause: 1 package(s)

npm Dependencies

See full report in workflow artifacts

Allowed Licenses: MIT, Apache-2.0, BSD-*, ISC, CC0-1.0, MPL-2.0
Forbidden: GPL, AGPL, SSPL (strong copyleft)