feat: support wallet password file for unattended operation

[NetteEddy]

Description

The wallet currently requires either an interactive password prompt or the
--pass command-line option. This makes unattended operation difficult, while
--pass exposes the wallet password through the process command line.

This change adds support for the EPIC_WALLET_PASSWORD_FILE environment
variable. When set, the wallet reads the password from the referenced file.

Password selection priority is:

1. --pass
2. EPIC_WALLET_PASSWORD_FILE
3. Interactive password prompt

Trailing CR/LF characters are removed from the password file so that files
ending with a normal newline can be used without modifying the password.

Existing interactive behavior remains unchanged.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Chore (miscellaneous changes e.g. modifying .gitignore)
• Build (Affect build components like build tool, ci pipeline, dependencies, project version)
• Docs (documentation update)

How Has This Been Tested?

• cargo test --release password_from_file -- --nocapture
• cargo build --release
• Tested epic-wallet listen -m http -n with the password supplied through EPIC_WALLET_PASSWORD_FILE

Relevant notes

The existing --pass option keeps precedence over the environment variable.
If EPIC_WALLET_PASSWORD_FILE is not set, the wallet continues to use the
existing interactive password prompt.

[who-biz]

Thanks for the PR. will review later - we are very busy atm

Appreciate concise PR, and the tests.

[blacktyger]

Blocking issue: parse_init_args calls prompt_password_confirm() and never looks at
the variable, and prompt_password_stdout turns a read error into "". So
EPIC_WALLET_PASSWORD_FILE=… epic-wallet init with no TTY prints "completed
successfully", prints a recovery phrase, and creates a wallet that opens with an empty
password. I confirmed the wallet then opens with an empty password and not with the
file's contents. The empty-password behaviour pre-dates this PR, but #127 is what makes
an operator run init unattended in the first place. Honour the variable in init and
change_password, and fail instead of defaulting to "".