Fix crashes caused by regex characters in email signature - #3252

Problem

Email signatures were dynamically converted into regular expressions while comparing and replacing draft content.

Signatures containing regex metacharacters, such as unmatched parentheses, could cause a PatternSyntaxException during draft autosave. Similar unsafe regex handling was also present when detecting or replacing Gmail alias signatures.

Solution

• Treat signatures as literal text when comparing draft content.
• Escape alias signatures before using them in multiline regex matching.
• Replace matched signatures by range so replacement characters such as $ and \ remain literal.
• Extract alias signature matching and replacement into a testable utility.
• Add regression tests covering signatures with regex metacharacters.

Testing

• Added unit tests for draft signature removal.
• Added unit tests for alias signature matching and replacement.

close #3232

Tests (delete all except exactly one):

• Tests added or updated

To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities