fix(input): support CJK editing over SSH (width-aware backspace + cursor movement)

[FrostMiKu]

Problem

When connecting over SSH, editing input containing CJK (Chinese/Japanese/Korean) characters was broken:

• Backspace left a half-glyph on screen. A wide character occupies 2 terminal columns, but the erase only cleared 1 column (\b \b), so deleting a Chinese character left the left half of the glyph stuck on the line.
• The cursor couldn't be moved within the line. There was no cursor model in the input buffer — it was append-only / delete-from-end. Left/Right/Home/End/Delete keys were not handled at all, so mid-line editing was impossible.

Fix

• Cursor model. Add a Cursor byte-offset to ClientInput. CleanserInputHandler now inserts at the cursor (instead of always appending) and deletes the rune before it; AnsiHandler moves it with the left/right/Home/End/Delete keys.
• Width-aware display. Echo and erase use runewidth so wide characters occupy the correct number of columns. Backspace now emits the right number of backspaces + clear-to-EOL instead of the single-column \b \b. Cursor moves (left/right/Home/End) move by the display width of the character they cross.
• History fix. Up/down history recall previously erased the screen by byte count (wrong for multibyte chars); it now erases by display width and keeps the cursor consistent.
• Consistency. Anywhere the buffer is resized (history recall, signal shortcuts, login submit, prompt redraws in main.go) now keeps Cursor valid.

Scope / unchanged behavior

• Only server-side-echo clients (SSH and raw telnet) get the new editing — they rely on the server for echo and line rendering.
• Local-echo clients (Mudlet, web client) are untouched; they manage their own input line.
• Masked (password) fields stay linear; no real characters are echoed and cursor editing is disabled there.

Verification

End-to-end over a real SSH connection (with a pty), capturing exactly what the server sends back to the terminal:

Action	Server output	Meaning
type 你好	raw UTF-8 bytes	displays correctly
Backspace	ESC[2D ESC[0K	back 2 cols + clear → full-width char erased (was \b \b = 1 col)
Left	ESC[2D	back 2 cols (crosses a wide char)
Right	ESC[2C	forward 2 cols
Home	ESC[4D	back 4 cols (width of 你世) → start

Tests added:

• Unit tests for width calc, key matching, mid-line insert, mid-buffer backspace.
• Integration tests that register a capturing connection and assert on the actual terminal bytes emitted for width-aware backspace, mid-line insert, and left-arrow movement.
• Existing TestCleanserInputHandler_UTF8Backspace / _NoBackspace updated to set Cursor (required by the new model).

make validate, go build ./..., and the inputhandlers / connections / term test suites pass. (internal/templates/TestRenderPanel_MaxWidth_AnsiTagsHandled fails on master independently of this change.)

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