* fix(security): escape backslashes in calendar bg-image CSS url()
The calendar event-background CSS escaped ' -> \' for a bg: image URL but
not backslashes first. Inside a single-quoted url('...'), \ is the CSS
escape char, so a URL value ending in/containing a backslash escapes the
closing quote and breaks out of the string, injecting arbitrary CSS. The
bg:<url> value is per-event and CalDAV-syncable, hence untrusted (CodeQL
js/incomplete-sanitization).
Add a single canonical _cssUrlEscape() in calendar/utils.js that escapes
backslashes FIRST, then quotes, and route all four sinks through it:
calendar.js:416 / :1263 (the flagged #463/#464), the event-form preview
(:2931), and _calBgCss() in utils.js — the latter two share the identical
bug but were unflagged. Output is byte-identical to the old escaping for
legitimate URLs (which contain no backslashes); only malicious input differs.
Resolves CodeQL js/incomplete-sanitization #463, #464.
* fix(security): route remaining calendar bg url() sinks through _cssUrlEscape
Review (vdmkenny) flagged that the centralization missed an injectable
sibling sink: the edit-form color-picker swatch (calendar.js:2856) built
`url('${url}')` from `existing.color` (a CalDAV-syncable, untrusted `bg:`
value) raw, then interpolated it into `style="background:..."` via innerHTML
- the same `'`/`\` breakout class as the sinks already fixed. The custom-dot
preview (:2953) was likewise raw (non-exploitable - a CSSOM `.style`
assignment of a URL the current user just picked - but it broke the invariant).
Route both through `_cssUrlEscape`, and normalize the two pre-escaped-variable
sites (_calItemBgStyle, _renderWeek) to the same inline form so all five
url() interpolations in calendar.js follow one rule. Add a whole-file
invariant test asserting every `url('${...}')` calls `_cssUrlEscape` - this
catches a future missed sink, the exact failure mode here. Behavior-identical
for legitimate URLs (no visual change).
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