refactor(themes): centralize sidebar width via --width-aside token (#8761)
## Summary
`--width-aside` already existed in `base-theme/frss.css` but only as a fallback inside the narrow-viewport rule from #8749. Desktop `.aside`, `#nav_entries`, `.reader .aside.visible`, and `.header > .item.title` hardcoded `300px` independently. This PR makes `--width-aside` the canonical token for the navigation drawer / left column width, so theme overrides apply everywhere consistently.
## Changes
`p/themes/base-theme/frss.css`:
- Define `--width-aside: 300px` in `:root`.
- `.aside`, `#nav_entries`, `.reader .aside.visible`, `.header > .item.title`: replace hardcoded `300px` with `var(--width-aside)`.
- Narrow-viewport `.aside.visible`: drop the now-redundant `320px` fallback in `min(...)`.
`p/themes/Swage/swage.css`:
- Remove the now-redundant `width: var(--width-aside)` overrides on `.aside`, `#nav_entries`, and `.header > .item.title` (inherited from base now). Swage's `--width-aside: 231px` declaration stays in place, so `#nav_entries`, the header `.title` block, and the reader-view sidebar now resolve to 231 via the token (previously stuck at 300 from base hardcodes), aligning them with Swage's other column elements.
RTL mirrors regenerated via `npm run rtlcss`.
## Why 300px
300px is the existing FreshRSS desktop sidebar value, so default themes see no visible change. The narrow-viewport cap previously used 320 as a fallback, but on actual phones the `calc(100vw - 56px)` cap dominates, so the change has near-zero practical effect on phone UX. Tablet and narrow-desktop sidebars become slightly narrower, more in line with typical desktop conventions.
## Tested
- Origine, Flat, Swage at desktop widths
- Origine and Swage at narrow viewports: sidebar slide, open, close
- Reader view in Origine and Swage
- Header title block: Swage 231, default themes 300
Co-authored-by: Bjørn A. Andersen <polybjorn@users.noreply.github.com>
polybjorn