feat: add Fable 5 to Claude model picker

[buallen]

Summary

• Adds claude-fable-5 to getClaudeModelModes() in packages/happy-app/sources/components/modelModeOptions.ts so the new model shows up in the mobile/desktop app's Claude model selector
• Adds claude-fable-5 to the MODELS array in packages/codium/sources/plugins/anthropic/index.ts so codium's Anthropic plugin lists it in its picker

Opus 4.8 was already added in #1363 / a8a4008, but Fable 5 wasn't, so users currently can't pick it from the UI.

Design notes

• Uses the full model id claude-fable-5 as the option key (rather than a short fable alias). The SDK transparently passes model through (runClaude.ts → Agent SDK), but fable as an alias hasn't been confirmed in the SDK yet, so the full id is safer. Once the SDK gains a fable alias, this can be normalized to match the opus / sonnet / haiku entries.
• No server-side change is needed — verified that packages/happy-wire/src/messageMeta.ts defines model: z.string().nullable().optional() (open string), and packages/happy-server does not hard-code any Claude model id.

Out of scope

• packages/happy-cli/src/utils/pricing.ts — Fable 5 pricing isn't published yet; leaving for a follow-up so we don't bake in guessed numbers.
• packages/codium/sources/agents/catalog.ts — only carries abstract opus / sonnet aliases, no per-version entries to maintain.

Closes #1380

Test plan

• Open the Happy app, start a Claude session → model picker shows "fable 5" between "default model" and "opus 4.8"
• Open codium → Anthropic plugin model picker lists "Fable 5" at the top
• Select Fable 5 in either UI → next user message round-trips through the Agent SDK without a schema/validation error

🤖 Generated with Claude Code