fix: use the output buffer for Serial Monitor select-all and copy

[iblackscreen]

Motivation

In IDE 1.x users could click into the Serial Monitor, press Ctrl+A and then Ctrl+C to copy the
whole output. In IDE 2.x this is not possible:

• The output is virtualized with react-window, so only the visible rows exist in the DOM. Mouse
  selection plus copy can only capture roughly one screen of data (Only the visible data can be copied from the serial Monitor #1081).
• Ctrl/Cmd+A is handled by the global core.selectAll keybinding, which selects the surrounding
  document instead of the monitor output (Missing "Select All" capability for Serial Monitor #812).
• Copied content arrives as formatted HTML, which pastes badly into spreadsheets (Content copied from Serial Monitor output is formatted text #2093).

This matters in education: schools and universities use Arduinos as low-cost measurement
instruments in physics classes, and copying a recorded measurement series out of the Serial
Monitor is a routine step there (several comments on #812 describe classroom use).

#2718 added a Copy Output toolbar button that copies the internal line buffer. This PR connects
the same buffer to the keyboard workflow users expect from IDE 1.x.

Change description

• Click into the Serial Monitor output and press Ctrl/Cmd+A: the output area is selected. This is
  a new serial-monitor-select-all-output command, bound via a keybinding context that is only
  enabled while the output area has focus. The global select-all behavior everywhere else,
  including the monitor's message input field, is untouched.
• Ctrl/Cmd+C then copies the complete buffered output as plain text, with the same null-character
  sanitizing as Copy Output. The shared logic is extracted into linesToPlainText in
  monitor-utils.ts.
• Copying a manual mouse selection inside the output now writes plain text instead of formatted
  HTML to the clipboard (Content copied from Serial Monitor output is formatted text #2093).
• The select-all state is cleared on click or when the output area loses focus. While it is
  active, incoming messages keep the selection alive.
• The focusable output area shows a themed focus outline, so keyboard users can see that the
  shortcut targets it.

Other information

• The copied "select all" content is the raw buffer without timestamps, identical to the
  Copy Output button. A manual mouse selection copies what is rendered, including timestamps if
  they are enabled. In both cases the clipboard receives plain text only.
• Mouse selection is still limited to the rendered rows: lines that are scrolled out of view do
  not exist in the DOM, so a drag selection cannot reach them. Selecting everything is what
  Ctrl/Cmd+A is for.
• feat(serial-monitor): replace virtual list with textarea for native text selection #2885 addresses the same issues by replacing the virtualized list with a read-only textarea.
  That gives native selection, but it re-renders the whole buffer on every message batch, which
  is the problem the virtualization was introduced for, and it loses the timestamp rendering.
  This PR keeps the virtualized list and the timestamps and only wires the existing buffer to the
  expected shortcuts.

Fixes #812
Refs #1081, #2093

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