Default tab colors invisible on light backgrounds (white-on-white)

[melvincarvalho]

Symptom

Default `tabColor` is `rgba(255,255,255,0.5)` and `tabActiveColor` is `rgba(255,255,255,0.9)`. On any app with a light background (the bulk of the modern ecosystem — solid-apps.github.io's apps, third-party LOSOS apps), the tab text is invisible. Same for the `pane-tabs` border which defaults to `rgba(255,255,255,0.08)`.

The losos.org `vcard` example already works around this with per-app CSS:

```css
button.pane-tab { color: #888 !important; font-weight: 500 !important; }
button.pane-tab[aria-selected="true"] { color: #667eea !important; font-weight: 600 !important; }
```

The fact that the canonical demo overrides the defaults via `!important` is the strongest signal that the defaults are wrong for the actual usage pattern.

Reproduction

Open any of the bundled examples (e.g. `examples/dashboard/`) — tabs at the top are barely visible against the white background. The active-tab purple underline is the only visible cue that there are tabs at all.

Root cause

`losos/shell.js` lines 123-128 + 142:

```js
var tabColor = opts.tabColor || 'rgba(255,255,255,0.5)'
var tabActiveColor = opts.tabActiveColor || 'rgba(255,255,255,0.9)'
...
tabBar.style.cssText = '...border-bottom:1px solid rgba(255,255,255,0.08);...'
...
tab.style.borderBottomColor = opts.accentColor || '#7c3aed'
```

All three colors assume a dark background.

Proposed fix

Switch defaults to colors that work on both light AND dark backgrounds:

```diff

• var tabColor = opts.tabColor || 'rgba(255,255,255,0.5)'
• var tabActiveColor = opts.tabActiveColor || 'rgba(255,255,255,0.9)'

• var tabColor = opts.tabColor || 'rgba(127,127,127,0.55)'
• var tabActiveColor = opts.tabActiveColor || (opts.accentColor || '#7c3aed')

• tabBar.style.cssText = 'display:flex;gap:0;border-bottom:1px solid rgba(255,255,255,0.08);overflow-x:auto;...'

• tabBar.style.cssText = 'display:flex;gap:0;border-bottom:1px solid rgba(127,127,127,0.18);overflow-x:auto;...'
  ```

Three-line change. Behaviour:

• Inactive tab: medium gray with transparency — readable on light or dark background. Translucent enough to feel secondary.
• Active tab: defaults to the existing accent color (`#7c3aed` / customisable via `opts.accentColor`). Already used for the bottom-border underline, so visually consistent. Removes the redundant tabActiveColor parameter from the default path.
• Tab bar border: same gray-with-transparency idea — visible on light, near-invisible on dark.

Apps that pass explicit `opts.tabColor` / `opts.tabActiveColor` continue to override exactly as before. Apps overriding via CSS `!important` (vcard) continue to win regardless.

Size impact

Tested locally:

	raw	gzip
Before	10,462	3,340
After	10,471	3,336
Delta	+9	−4

Effectively zero. The fix doesn't grow shell.js (gzip actually shrinks by 4 bytes due to the new color values compressing slightly better).

Tested

Verified locally against `examples/dashboard/` (white background) and `drop-demo.html`: both render readable tabs with the purple-accent active state.

Happy to send a PR if accepted. Wanted to confirm the approach (and the `tabActiveColor` consolidation onto `accentColor` — a small API simplification) before pushing.