fix(dashboard): extend the narrow band to the width it still clipped

[rmyndharis]

The band that swaps the chat panes ended one pixel short. The room is viewport - 646 with the navigation expanded and the composer's minimum is 244px, so the first viewport at which two panes fit is 890, not 889. At exactly 889px the two-pane layout was kept and one pixel column of the send button fell outside the panel's clip.

Root cause of the off-by-one

The measurement that set the bound compared the send button's right edge against .chats-layout's border box. overflow: hidden clips at the padding box, and that element carries a 1px border, so every threshold derived that way was low by exactly one. Three published figures inherit it, and all three are corrected here.

Figure	Published	Measured
Band upper bound	888px	889px
Composer clip started below	1014px	1015px
Phone widths affected	402px and narrower	403px and narrower

What changed

• @media (min-width: 769px) and (max-width: 888px) becomes 889px.
• The block comment stated "from 889px the room fits the composer again" and claimed the bound tracks the composer's minimum. Neither was true; it now states the arithmetic it is derived from, 646 + 244 - 1.
• The two changelog figures above are corrected in place.

Impact

Measured against the built bundles (index-*.css plus Chats-*.css from npm run build), not the source file, with the real webfont. sendVsClip is the send button's right edge against the layout's padding box, the actual clip edge; negative is inside.

Viewport	Before	After
888px	column, −24.0	column, −24.0
889px	row, layout overflow 1px, +1.0 (clipped)	column, overflow 0, −24.0
890px	row, 0.0	row, 0.0
891px	row, −1.0	row, −1.0

Navigation collapsed is unchanged at 888, 889 and 890, as it was before: that state never clipped.

Verification

• A 53 width sweep from 769px to 1440px, one pixel apart across 881..900 and wider steps outside it, asserting per row that the rendered viewport matched the requested one and that the webfont was loaded: 0 empty rows, 0 widths where the layout overflows or the send button crosses the clip. Before this change that sweep reports 889px.
• Boundary table above reproduced on the built bundle, so it reflects what ships rather than the source file.
• Dashboard lane: lint, format:check, typecheck, i18n parity, unit tests. Docs lane: 264 tests. The page-CSS scope gate passes, 11 of 11.

Refs #1422