feat: add select-all checkbox and lift selection state to parent

[RobinAngele]

Depends on: #12899

Refs #4285
Refs #7880
Refs #6070
Refs #7276
Refs #11526

Follow-up: #12901 adds filter-based mass selection

Summary

Adds a "Select all X messages" checkbox above the envelope list and lifts
envelope selection state from individual EnvelopeList instances to the
Mailbox parent, enabling cross-group shift-click range selection and
global select-all/unselect-all.

Together with #12901 this fully closes #4285 and #7880.

Changes by file

Mailbox.vue

• Select-all checkbox using NcCheckboxRadioSwitch at the top of the list
• Shows "Select all N messages" with the current visible page count
• Hidden once selection mode is active (multiselect header takes over)
• margin-top: 8px so it doesn't sit flush against the sticky search bar
• selection array in data — single source of truth for selection state
• selectAll(), unselectAll() methods for global control
• flatEnvelopeList, selectMode, allSelected computed properties
• onUpdateSelection(), onSelectRange(), getGroupFlatIndex() methods
• Passes selection prop down to EnvelopeList children
• Listens for update:selection and select-range events from children
• Resets selection on mailbox or filter change
• .select-all-bar CSS

EnvelopeList.vue

• selection prop (type Array, replaces local data().selection)
• flatIndex prop (type Number, for cross-group shift-click indexing)
• Watch on selection prop with immediate: true to sync flags.selected
• emitLocalSelection() method — emits update:selection to parent
• Shift-click now emits select-range with global flat indices
• Deleted local selection from data() and setEnvelopeSelected mutations

How to test

1. Open a mailbox → "Select all N messages" checkbox visible at top
2. Click it → all visible messages selected → multiselect header appears
3. Shift-click across messages in different date groups → range selected
4. Change mailbox or filter → checkbox resets to unchecked
5. Unselect all → checkbox reappears

Screenshots