Vi-modal editing for the raw request textareas

Started a broader pass to close the gap with Burp/ZAP/Caido (feature
comparison researched, tracked in PLAN.md) and to fully support vi
bindings as asked. This commit is the vi-bindings piece.

Checked the actual bubbles library before writing anything: table and
viewport already ship full vi navigation by default (h/j/k/l, ctrl+u/
ctrl+d, g/G) - nothing to build there. textarea and textinput are
Emacs-style with no vi support at all, and textarea's own ctrl+p
("previous line") was silently shadowed by the ctrl+p shortcut I'd
bound for Intruder's marker insertion - a real bug from the last
session, fixed here by moving it to ctrl+g.

vimode.go adds viTextarea, wrapping textarea.Model with a normal/insert
modal layer. Vi commands are translated into the underlying textarea's
own existing keybindings (a synthesized "alt+right" for `w`, "ctrl+k"
for `d$`, etc.) and reuse its tested cursor/line logic rather than
reimplementing text manipulation - this is a front-end over textarea,
not a parser. Covers what's actually used constantly: h/j/k/l, 0/$,
w/b, x, i/a/I/A/o/O, dd/yy/p/P, dw/d$/d0, gg/G, esc-to-normal (esc
never leaves the view while still in insert mode, matching real vi).
Not attempted: registers beyond one yank slot, visual mode, ex
commands, macros, counts. No undo, since textarea itself has none.
Starts in normal mode on focus, per vi convention - not insert.

Replaces the three textarea.Model fields (Repeater's request editor,
Intruder's template and payload editors) with viTextarea, and adds a
vim-style mode indicator ("-- NORMAL --" / "-- INSERT --") to both
views, without which modal editing is unusable - there'd be no way to
tell which mode you're in.

Verified live via a real session: normal-mode letters don't leak into
the buffer as text, gg/x/dd/yy/p all produce the correct edits in
sequence on a real captured request, i enters insert mode and typing
works, esc from insert correctly drops to normal without leaving the
view, a second esc then backs out, and ctrl+g inserts a marker without
colliding with anything in the vi command set.

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