fix(external-sources): i18n, alignment, and visual polish for config UI

[limityan]

Summary

Fixes multiple i18n, alignment, and visual clarity issues in the External AI Applications settings page.

Changes

Bug fixes

• Ecosystem mode Select not closing: When the master toggle is off, ecosystem mode dropdowns still showed the previous value instead of "disabled". Now maps to disabled when the master toggle is off, matching capability access behavior.
• Raw English diagnostic messages leaking to UI: Hook diagnostics, source diagnostics, and operation errors showed raw backend English strings. Removed all raw diagnostic.message rendering; only show the i18n category text.
• Raw diagnostic codes shown to users: Codes like opencode_default_permission_semantics_not_imported were displayed inline. Removed from all rendering locations.

Structural changes

• Removed ExternalHooksPanel from page (read-only, no decision impact)
• Moved policy/master toggle to top of page
• Removed catalog-level diagnostics section (redundant with per-source)
• Filtered info-severity hook diagnostics (not actionable)
• Flattened agent diagnostics: no nested collapsible, only show nextStep for blocking items

i18n improvements (zh-CN / en-US / zh-TW)

• Rewrote all hook diagnostic texts — removed jargon
• Rewrote all agent diagnostic texts — shorter, plain language
• Rewrote MCP empty state — removed incomplete JSON example, clarified purpose
• Added operationErrors.errorCode and hooks.diagnosticCode keys

Visual polish

• Removed gray box backgrounds that looked muddy in light theme
• Added chevron indicators to all expandable details areas
• Added left-border grouping and top-border separators
• Fixed alignment: all content shares same left edge within sections
• Fixed chevron-text spacing
• Cleaned up unused SCSS rules

Verification

Command	Result
pnpm run type-check:web	Pass
pnpm run i18n:audit	Pass (0 warnings)
pnpm run theme:color-audit:all	Pass
ExternalSourcesConfig tests	47/47 pass
ExternalHooksPanel tests	13/13 pass