feat(message): show grouped-message timestamps and clickable edit history

[Th-Underscore]

Note: #1942 adds an always-on timestamps setting; this PR takes a complementary hover-only approach and will be rebased on top of dev after that merges.

Description

Two related timeline polish items:

• Hover timestamps on grouped messages. Consecutive messages from the same sender are collapsed into one block; hovering the block now shows the send time of each message as a small footnote in the left gutter (timeHourMinute, matching the existing "reacted with" underscript styling). The most recent message already shows its time - this surfaces the hidden ones.
• Interactive "(edited)" indicator. Hovering (edited) shows a tooltip with the edit timestamp(s) (newest last, via getEventEdits). Clicking it now opens the message's version history - the exact same modal as right-click -> "Version History". Keyboard accessible (Enter/Space); propagation is stopped so surrounding message handlers don't fire.

Purely cosmetic interaction layer; no data changes. Related to #1942 (which shows timestamps in the message header) - this takes a lighter-weight approach that only surfaces times on hover, keeping the timeline uncluttered.

Tested on the desktop app, Ubuntu 24.04.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation - .changeset
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

I designed and directed the features and tested them in the GUI; the code itself was AI-generated.

The timestamp footnote reuses the reaction underscript's typography scale and renders from the same collapse-grouping pass the timeline already computes. Edit timestamps thread through an editTimestamps prop from RenderMessageContent into the text renderers so MessageEditedContent can build tooltip strings. Click-to-history reuses the context menu's setModal({ type: ModalType.EditHistory, ... }) call verbatim rather than reimplementing it, gated on room && mEvent being present so non-timeline render sites stay inert.