Implement bracket pairing and handling for user input

[ShingZhanho]

This pull request implements auto-completion features for LaTeX input in the game.
All auto-completions apply to the #user-input textarea in the game.
Below is a summary of the implemented features, with examples of the auto-completion behavior.

Bracket pair auto-close

Typed	Result	Cursor
(	()	between
[	[]	between
{	{}	between

• Selection wrap: with text selected, typing (, [, or { wraps the selection in the pair.
• Skip-over: typing ), ], or } when that character is already immediately after the cursor moves the cursor past it instead of inserting a duplicate.
• Paired backspace: pressing Backspace between a matched pair deletes both brackets.

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\left/\right delimiter pairs

Typed	Result	Cursor
\left(	\left(\right)	between
\left[	\left[\right]	between
\left{	\left{\right}	between
\left\{	\left\{\right\}	between
\left|	\left|\right|	between
\left\|	\left\|\right\|	between
\left<	\left<\right>	between

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Escaped bracket pairs

Typed	Result	Cursor
\{	\{\}	between
\|	\|\|	between

| alone does not trigger auto-completion.

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Named delimiter pairs

Typed	Result	Cursor
\langle	\langle\rangle	between
\left\langle	\left\langle\right\rangle	between
\lfloor	\lfloor\rfloor	between
\left\lfloor	\left\lfloor\right\rfloor	between
\lceil	\lceil\rceil	between
\left\lceil	\left\lceil\right\rceil	between
\lvert	\lvert\rvert	between
\left\lvert	\left\lvert\right\rvert	between
\lVert	\lVert\rVert	between
\left\lVert	\left\lVert\right\rVert	between

\langle completion is incremental: \lang first inserts \lang\rang, then completing to \langle automatically upgrades the closing tag to \rangle (or \right\rangle when preceded by \left\).

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Environment auto-close

Typed	Result	Cursor
\begin{align}	\begin{align}\end{align}	between

• { after \begin is not auto-closed — the user types the environment name freely and closes with }.
• Typing } to close \begin{envname triggers the completion: inserts }\end{envname} and places the cursor between the \begin and \end blocks.

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Navigation

Key	Action
Tab	Jumps cursor past the next } in the text. If the cursor is immediately before \end{...}, jumps past the entire \end{envname} tag instead.