feat: sticky ancestor tabs

[the-nelsonator]

Closes #2368

Summary

Adds an optional sticky ancestor tab section to tabs panels. When enabled, the ancestor chain (parent → grandparent → …) of the active tab is pinned to the top of the panel as you scroll, so you never lose track of where you are in a large tree. Clicking a sticky row activates that ancestor (and scrolls to it).

Controlled by settings Tabs > Tabs tree > Sticky ancestor tabs (off by default) and Sticky ancestor tabs depth limit (default 3).

Before scroll down	After scroll down

Demo: https://pixeldrain.com/u/pcLTdKNC

Behaviour

• Pins the active tab's ancestors to a fixed/sticky section at the top of the panel that contains the active tab. Clicking activates the ancestor tab.
• Only ancestors that are actually hidden are shown — i.e. scrolled above the viewport top or covered by the sticky rows already pinned above them. Scrolled fully to the top → nothing is pinned.
• The pinned rows are real tabs: favicons, context menu, drag, close/expand, container colours, and active styling all work exactly like normal tabs.
• Depth (amount of parents above active tab) is capped by a configurable limit (default 3, options 1–5 / none) so deep trees don't fill the viewport.
• Honours tree layout: the feature and its sub-option are inactive when "Tabs tree structure" is off, and hidden during search.

Implementation notes

Compute — Tabs.calcStickyTabs(panel) (src/services/tabs.fg.scroll.ts):

• Starts from Tabs.byId[Tabs.activeId]; bails unless the active tab is in this panel and not pinned.
• Walks the parentId chain (root → active).
• Filters to hidden ancestors by accumulating the sticky-stack height: walking root→active, include while idx*rowH - scrollTop < stackBottom, then stackBottom += rowH. Each pinned row pushes the cutoff down by one row height (it hides the tab beneath it). Short-circuits at the first fully-visible ancestor since rows are monotonically ordered.
• Applies the depth cap (keeps the deepest N) and writes panel.reactive.stickyTabIds only when it changes.

Recompute triggers:

• Scroll — passive scroll listener on the panel's scroll element.
• Active-tab switch — pushed from onTabActivated (tabs.fg.handlers.ts) for the new panel and the panel the active tab left (Tabs.activeId isn't Vue-reactive).
• Structure / toggle — deep watch on visibleTabIds + the feature setting in panel.tabs.vue, plus an initial compute on mount.

Rendering:

• New sticky-tabs.vue overlay renders the real tab.vue component with a new :sticky prop, one per id.
• tab.vue gains sticky?: boolean: omits the id="tab<id>" attribute (no duplicate DOM ids), adds a -sticky class, and — crucially — does not write the shared element refs (titleEl/favImgEl/favSvgUseEl/flashFxEl) onto the tab object, which would hijack the real row's imperative favicon/title/flash updates. The favicon is instead rendered into the clone's own elements via the new Tabs.renderFaviconInto(tab, imgEl, svgUseEl) (extracted from renderFavicon); onError falls back locally too.
• Styled via src/styles/sidebar/sticky-tabs.styl: a zero-height position: sticky; top: 0 anchor inside the scroll viewport (same mechanism as the existing new-tab bar), so it takes no flow space (no layout shift) and aligns automatically. Solid backdrop + a separator shadow under the lowest row.

Files changed

• Settings: types/settings.ts, defaults/settings.ts, page.setup/components/settings.tabs.vue, _locales/dict.setup-page.ts
• Panel state: types/sidebar.ts, defaults/panels.ts
• Logic: services/tabs.fg.scroll.ts (calcStickyTabs), services/tabs.fg.ts (renderFaviconInto), services/tabs.fg.handlers.ts (recompute on activation)
• UI: sidebar/components/sticky-tabs.vue (new), sidebar/components/tab.vue (sticky prop), sidebar/components/panel.tabs.vue (overlay + triggers)
• Styles: styles/sidebar/sticky-tabs.styl (new) + import in styles/sidebar/sidebar.styl

[the-nelsonator]

> sidebery@5.5.2 lint.types
> vue-tsc --noemit

Error: src/defaults/mocks.tabs.fg.ts(67,5): error TS2353: Object literal may only specify known properties, and 'preview' does not exist in type 'ReactiveTabProps'.
Error: src/defaults/mocks.tabs.fg.ts(109,23): error TS7053: Element implicitly has an 'any' type because expression of type '"folded" | "hidden" | "url" | "title" | "isParent" | "autoUnloadFoldedTimeout" | "invisible" | "parentId" | "panelId" | "prevPanelId" | "lvl" | "sel" | "selLock" | "updated" | ... 54 more ... | "groupId"' can't be used to index type 'MTab'.
  Property 'splitViewId' does not exist on type 'MTab'.

Not related to these changes