fix(tui): decode percent-encoded file URIs in initial prompts

[ca0fgh]

Issue for this PR

Closes #15561

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Fixes percent-encoded file:// path rendering in TUI initial prompts.

• Adds decodeFileUriForDisplay and decodeFileUrisInText to packages/util/src/path.ts.
• Decodes percent-encoded Unicode path segments (for display only) while preserving query/hash.
• Applies decode at TUI prompt hydration points:
  • packages/opencode/src/cli/cmd/tui/routes/home.tsx
  • packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
• Guardrail: only decodes when initialPrompt.parts.length === 0 to avoid shifting extmark/source offsets for prompts that include structured parts.

How did you verify your code works?

• Ran:
  • bun run typecheck in packages/util
  • bun run typecheck in packages/opencode
• Reproduced with a path containing Chinese characters (for example 测评):
  • before: file:///.../%E6%B5%8B%E8%AF%84/...
  • after: file:///.../测评/...

Screenshots / recordings

N/A (TUI text rendering fix)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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