Assign Folders picker now keeps action buttons visible in Mail

This QA release promotes a single Mail UI fix (PR #1037) from dev to qa. When assigning folders to selected mail, the folder picker now expands the dialog in place instead of floating over it, so the Cancel and Assign Folders buttons always stay visible.

In the Assign Folders bulk-action modal, the folder dropdown previously opened upward to avoid covering the footer buttons. An upward-flipping list looked unanchored and non-standard, while the only other option (opening downward as a floating overlay) covered the Cancel / Assign Folders actions. Neither behavior gave users a clean, predictable way to pick folders.

Users assigning folders to one or more mail items get a clearer, more standard experience: the picker opens downward and grows the dialog so the confirmation buttons remain in view. Because the list uses a fixed height, typing to filter folders no longer resizes the dialog or makes it jump around as it re-centers.

• Adds a new INLINE option to the shared MultipleSelector MULTIPLE_SELECTOR_DROPDOWN_PLACEMENT enum, alongside existing DOWN / UP.
• INLINE mode renders the option list in normal document flow (relative, softer shadow-sm) instead of as an absolutely-positioned overlay, so it pushes following content (the footer) down rather than covering it.
• Inline lists use a fixed height (defaulting to 240px) so filtering doesn't resize the container; floating DOWN/UP modes keep their prior maxHeight behavior.
• AssignFoldersToMailModal.tsx switches its folder picker from UP to INLINE; doc comment updated accordingly.
• Net change is small (~2 files, +43/-16). Default DOWN and other UP consumers of MultipleSelector are explicitly unchanged.

Low risk. The change is additive: a new placement mode plus one consumer opting into it; existing DOWN/UP callers retain their prior styling and positioning. Main risk is layout-specific — in very short viewports the in-flow list could grow the dialog beyond the screen. Mitigation: the fixed list height caps growth and the list scrolls internally; change was validated on dev before promotion.

• In Mail, select multiple items and open Assign Folders; confirm the picker opens downward and the Cancel / Assign Folders buttons stay visible.
• Type to filter the folder list and confirm the dialog does not resize, jump, or re-center.
• Scroll within a long folder list and confirm internal scrolling works without a horizontal scrollbar.
• Regression-check other surfaces using MultipleSelector with UP/DOWN placement to confirm their dropdowns are unchanged.
• Verify folder selection, deselection, and successful assignment still work end to end.