feat: add select-all checkbox, fix missing important icon and favorite/unfavorite logic

[RobinAngele]

Summary

Adds a "Select all X messages" checkbox above the envelope list, fixes a missing icon during bulk selection, and corrects broken favorite/unfavorite bulk action logic.

References

Ref	Title	How this PR relates
Closes #4285	Select all messages in mailbox	Implements the requested Select All checkbox
Closes #7880	Implement a "Select All" menu item	Provides Select All via persistent checkbox (always visible, no menu needed)
Refs #6070	Improve experience of multiselecting mails	Adds visible checkbox + shift-click range for much better discoverability
Refs #7276	Mass handling of items need numerous upgrades	Implements shift-click range selection and Select All as described
Refs #11526	Multiple multi actions headers per date group	Lays groundwork by centralizing selection state in parent

Changes

New feature: Select all checkbox

• Renders an NcCheckboxRadioSwitch at the top of the message list showing "Select all N messages"
• Hidden once selection mode is active (multiselect header takes over)
• Includes proper margin-top so the bar doesn't sit flush against the sticky search header

Selection state lifted to parent

• Moved selection state from individual EnvelopeList instances up to the Mailbox parent
• Enables cross-group shift-click range selection via flat envelope indexing
• Ensures consistent selection state across grouped envelope lists (Today, Yesterday, etc.)

Bug fixes

• Missing "Mark as important" icon — ImportantIcon was used in the template of EnvelopeList.vue but never imported or registered. The icon now renders correctly during bulk selection.
• Favorite/unfavorite not working — favoriteAll() and unFavoriteAll() computed favFlag by inverting a boolean check on the selection state. When all selected messages had the same state (e.g. all favorited), the computed flag would be true instead of false, making unfavorite a no-op. Fixed by using explicit favFlag: true / favFlag: false.

How to test

1. Open a mailbox with multiple messages
2. Click the "Select all N messages" checkbox → all messages should be selected
3. In the multiselect header, click Favorite (star icon) → all selected messages become favorited
4. Click Unfavorite (unfilled star) → all messages become unfavorited
5. With mixed selection, click Mark as important → the important icon should appear
6. Shift-click to select a range across date groups (e.g. Today → Yesterday)

Screenshots

Before	After
No select-all bar	Select-all bar with margin-top clearly visible
Missing important icon	Important icon renders in multiselect header
Unfavorite does nothing	Unfavorite correctly removes star from all selected

[ChristophWurst]

Thank you for your contribution!

On a high level the changes make sense, but please split them into individual PRs so that we can backport the fixes and have the features for main only.

If I'm not mistaken, the changes are very likely produced by an AI agent. Please direct it to the AGENTS.md file to follow the commit requirements.

Undo the funding change.

[RobinAngele]

Split into #newPR-A and #newPR-B per maintainer request @ChristophWurst

[ChristophWurst]

I meant fixes as plural, unless they are strongly connected.

[RobinAngele]

Split into two smaller PRs:

• 🔧 Bug fixes (backportable): fix: add missing ImportantIcon and correct favorite/unfavorite bulk actions #12899
  The bug fix PR only touches EnvelopeList.vue (+8/-8) and can be backported.
  (Only EnvelopeList.vue (+8/-8) — ImportantIcon + favorite/unfavorite)

• ✨ Select-all feature (main only): feat: add select-all checkbox and lift selection state to parent #12900
  The feature PR builds on top with Mailbox.vue (+147/-1) + EnvelopeList.vue extensions.
  ( Mailbox.vue + EnvelopeList.vue — selection state lifted to parent)

Okay like that?

A third PR to enable filter selection is here : #12901
🔍 Filter mass-select (main only, depends on #12900): #12901
SearchMessages.vue + MailboxThread.vue + Mailbox.vue — completes the feature

Merge order: #12899 → #12900 → #12901
Each PR declares its dependencies in the description.

@ChristophWurst