When box mode is active, reduce both the screen buffer and PTY size
by 2 in each dimension to reserve space for the border:

- window_pane_resize(): Reduce screen buffer to (sx-2, sy-2)
- window_pane_send_resize(): Report PTY size as (sx-2, sy-2)
- options_push_changes(): Resize screen/PTY when option changes

This ensures the shell receives the correct terminal dimensions and
content cannot be written in the border area.

Adjust content and cursor positioning when box mode is active:

- screen_write_set_client_cb(): Apply +1 offset to xoff/yoff for
  content positioning inside the border
- server_client_reset_state(): Account for box mode offset when
  calculating cursor position
- tty_window_offset1(): Account for box mode offset in view offset
  calculation

This ensures content is rendered inside the border area and the
cursor appears at the correct position.

Add screen_redraw_draw_active_box() to draw a complete box border
around the active pane when box mode is enabled:

- Draw styled border (using pane-active-border-style) for active pane
- Draw blank spaces for inactive pane border areas
- Skip normal pane separator borders when box mode is active
- Adjust pane content drawing to account for box mode offset
- Call box drawing on border redraws, window redraws, and pane redraws

The box border uses the configured pane-border-lines style for the
line drawing characters.

Add "box" as a valid choice for the pane-border-indicators window
option. Update the option description to mention the new box mode.

Update tmux.1 man page to document the new "box" value for
pane-border-indicators, explaining that it draws a complete box
around the active pane and that content is inset to prevent reflow.

When box mode is active, the separator columns and rows between panes
were not being cleared, causing old border characters to persist.
Each pane's box is drawn within its allocated space, but the layout
system places separator cells between panes that were left untouched.

Add code to clear:
- The separator column immediately to the right of each pane
- The separator row immediately below each pane
- The corner cell where separators intersect