refactor(tui): theme switching & loading reactivity

[nihil2501]

Issues

• Prepares [FEATURE]: Custom themes can use calculated system colors anomalyco/opencode#6322

Refactor

Remodels theme resolution reactivity with respect to this tuple:

• The user's requested theme
• The loading status of relevant async resources

This change also prepares for a feature: system-like custom themes

Fixes

• Theme picker list now updates on reload
• Warning toasts on errored
  • These toasts could easily be removed

Performance

The theme context now readies 10x faster in by-far the most common case: using one of the default themes. The speedup is achieved by not waiting on resources irrelevant to the requested theme before marking it as ready.
Timed using this patch that calls console.time inside createSimpleContext.

After = 3.558ms

...
[17:05:25] [LOG] '%s: %s' 'context:Theme' '3.558ms'
...

Before = 31.264ms

...
[17:04:19] [LOG] '%s: %s' 'context:Theme' '31.264ms'
...

Implementation

• 2x createResources
  • Custom theme loading
  • System palette detection
• These in turn are accessed in a createMemo returning this tuple:
  • The user's requested theme
  • The loading status of relevant async resources
• 2x createEffects
  • One to update the requested theme when the config generation increments
  • Another to redo theme resolution when our memoized tuple changes