Support custom AI gateway auth

[tigercraft4]

Summary

• add a Custom AI Coach auth-header selector for Bearer vs x-api-key on Apple and Android
• use the selected header for Custom chat and model-list requests while keeping Bearer as the default
• parse catalog-shaped custom /models responses and add Android fallback to max_completion_tokens

Closes #765

Verification

• xcodegen generate && xcodebuild -project Strand.xcodeproj -scheme Strand -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO build
• xcodegen generate && xcodebuild -project Strand.xcodeproj -scheme Strand -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO -only-testing:StrandTests/AIProviderModelListTests test
• JAVA_HOME=/opt/homebrew/opt/openjdk@21 ANDROID_HOME=$HOME/Library/Android/sdk ./gradlew --dependency-verification lenient compileFullDebugKotlin
• Android unit-test command was attempted, but this checkout currently fails compiling unrelated tests whose fake DeviceRegistryDao implementations do not implement setModel; app Kotlin compile passes.

[ryanbr]

Nice, clean cross-platform feature — thanks. The code is solid; I verified it fairly thoroughly and there's just one merge-order fix needed before this can land.

Code review — looks good

• Auth-header parity (Swift ↔ Kotlin): the new CustomAIAuthHeader/CustomAiAuthHeader enum (Bearer / x-api-key, default Bearer) matches on both platforms. applyCustomAuthHeader is applied to both the chat POST and the model-list GET, only for the Custom provider (no leakage into the OpenAI/Anthropic/Gemini paths), and it's keyless-guarded so local servers still get no auth header. Good.
• Security: key handling is unchanged and correct — still Keychain / encrypted store, still owner-gated so one provider's key never hits another's endpoint, and only the user-selected header carries it. The persisted values differ per platform ("bearer" vs "BEARER"), which is fine since this setting isn't part of the .noopbak contract.
• catalog[].models parsing: parity — both add a deduped catalog path alongside the existing data[].id path.
• max_completion_tokens retry: the Android side now mirrors Swift's existing retry-on-400 (Custom.swift), which is the right direction (Android catching up). Functionally equivalent; the heuristic wording differs slightly but that's harmless.
• Tests on both sides cover the catalog parsing.

Blocking: pt-PT Android coverage (branch-timing)

The green i18n check on this PR is stale — it ran before #769 made pt-PT a focus locale. Merging this onto current main and running the audit fails with pt-PT: 2 gap(s): the two new Android strings were added to values/de/es/fr but not values-pt-rPT (which didn't exist when this branched). The Apple .xcstrings already include pt-PT for the equivalents, so the Portuguese text exists — it just needs mirroring into Android.

Please add these two lines to android/app/src/main/res/values-pt-rPT/strings.xml:

<string name="l10n_coach_screen_key_header_3f2a9b10">Cabeçalho da chave</string>
<string name="l10n_coach_screen_use_bearer_for_most_local_servers_4429ab64">Usa Bearer para a maioria dos servidores locais; usa x-api-key para gateways que exigem a chave nesse cabeçalho.</string>

(Text lifted from this PR's own .xcstrings pt-PT entries; no apostrophes, so no escaping needed.) After that, python3 Tools/i18n_audit.py --full reports pt-PT missing=0 and the gate is clean.

Note

These are app-target changes with no default CI coverage, so the local Strand build + compileFullDebugKotlin you ran are the only compile validation — worth keeping in mind for the reviewer merging.