2420: move to groups/parent tabs via context menu

[the-nelsonator]

Relates to #2420 (doesn't implement the omnibox portion of the suggested changes)

Closes #619

Relates to #2089 (as a followup could add favorites star within these menus that makes it show up in Move To section)

Summary

Implements #2420: lets you move selected tabs into an existing tab group from the right-click Move to menu, instead of only via drag-and-drop. Scoped to the context menu only — the omnibox side of that issue is not included as I don't use the omnibox integration and am less familiar with it.

Adds default true boolean setting Tabs > Tabs Tree > Allow moving to parent tabs, when false limits to groups, useful if user thinks volume of parent tabs in menu is too much and just wants groups.

Demo

Moving to groups, cross-panel and same-panel:

move-to-group-basic.mp4

Moving to subgroup:

move-to-subgroup.mp4

Allow moving to parent tabs :

parent-tabs.mp4

What changed

Core logic

• src/services/tabs.fg.groups.ts (+142) — the index this feature runs on: groupTabIds/parentTabIds sets maintained incrementally (no full-list scans), getGroupsStruct() building the nested per-panel destination tree (groups always, parent tabs when the setting's on), revision-cached.
• src/services/tabs.fg.move.ts (+37) — moveToGroup() to drop a selection into a group/parent tab's branch (honors moveNewTabParent); hooks to keep the group/parent-tab index in sync on cross-window moves and detach.
• src/services/tabs.fg.handlers.ts (+10) / tabs.fg.ts (+5) / settings.fg.ts (+2) / sidebar.fg.ts (+2) — the remaining incremental-index hook sites: tab created/removed/reparented, tree recompute, tree-settings reset.

Menu option & UI

• src/services/menu.fg.options.tabs.ts (+92) — reworks moveToPanel: panels gain a submenu of their groups/parent tabs, selection-aware exclusion, icons (icon_group/icon_tab/favicon), and the new destination's onClick.
• src/services/menu.fg.ts (+45) — native (Firefox) context-menu mode now builds submenus recursively instead of one level deep.
• src/sidebar/components/popup.context-menu.vue (+70) — custom menu gets a navigable submenu stack (not just one level) and a dedicated > chevron button per row.
• src/styles/sidebar/popup.context-menu.styl (+40) — styles for the chevron button, including its hover background.

Setting

• src/types/settings.ts, src/defaults/settings.ts (+1 each) — new allowMovingToParentTabs: boolean, default true.
• src/page.setup/components/settings.tabs.vue (+7) — its toggle in the Tabs section.
• src/_locales/dict.setup-page.ts (+11) — label translated into all 9 locales (en/de/fr/hu/pl/ru/zh_CN/zh_TW/ja).